Vaccines contain a mix of ingredients, each with a specific job. The star ingredient is always the antigen, the piece of a germ that trains your immune system to recognize a future threat. Everything else in the vial exists to make that antigen work better, stay stable during storage, or remain sterile. Here’s what’s actually in there and why.
The Active Ingredient: Antigens
The core of every vaccine is one or more antigens. These are weakened or inactive parts of a virus or bacterium that trigger your immune system to build defenses without causing the actual disease. Different vaccines use different forms of antigens depending on what works best against a given pathogen.
Some vaccines use a whole virus that’s been killed or weakened so it can’t make you sick. The flu shot, for example, uses an inactivated virus. The MMR vaccine uses live viruses that have been weakened to the point where they replicate just enough to spark an immune response but not enough to cause illness. Other vaccines skip the whole organism entirely and use only a specific protein from the germ’s surface. Hepatitis B and HPV vaccines work this way, using proteins grown in yeast cells rather than the actual virus.
mRNA vaccines, like the COVID-19 vaccines from Pfizer and Moderna, take yet another approach. They deliver a genetic instruction that tells your own cells to make a single viral protein temporarily. Your immune system spots that protein, mounts a response, and remembers it for later. The mRNA itself breaks down within days.
Adjuvants: The Immune Boosters
Some vaccines include an adjuvant, an ingredient that strengthens the immune response so the vaccine works better. Not every vaccine needs one. Live vaccines that use weakened viruses typically generate a strong enough response on their own. But vaccines built from inactivated germs or isolated proteins often do.
Aluminum salts are by far the most common adjuvant. They’ve been used in vaccines since the 1930s, when researchers discovered they made diphtheria and tetanus vaccines more effective. Today, small amounts of aluminum hydroxide, aluminum phosphate, or similar compounds appear in dozens of vaccines, including those for hepatitis A, hepatitis B, HPV, pneumococcal disease, and tetanus. The amount of aluminum in a vaccine dose is tiny, far less than what you take in daily through food, water, and even breast milk.
Stabilizers That Protect the Vaccine
Vaccines are biological products, and like any biological material, they can degrade if exposed to heat, light, or the stress of freeze-drying. Stabilizers prevent that breakdown so the vaccine still works when it reaches your arm. Common stabilizers include sugars like sucrose and lactose, amino acids like glycine, and proteins like gelatin or human serum albumin. These ingredients are the same substances found naturally in food and in your body.
Gelatin is one of the more notable stabilizers because it’s a potential allergen for a small number of people. It appears in several live virus vaccines, including MMR, chickenpox, and shingles vaccines. Among all vaccine components, gelatin causes the most allergic reactions, though such reactions remain rare overall.
Buffers and Salts for pH Balance
Vaccines need to stay at a specific acidity level to remain effective. Buffers, usually simple salt solutions, keep the pH stable. Common ones include phosphate-buffered saline (a mix of sodium chloride and potassium phosphate), tromethamine (also called Tris), and sodium acetate. Sucrose often appears alongside these buffers, not as a sweetener but as a protective agent that shields vaccine components during freezing and thawing. These are the same kinds of salts your body already uses to regulate its own chemistry.
Growth Materials and Trace Residuals
To make a vaccine, you first need to grow the virus or bacterium in large quantities, then harvest and purify the antigen. The materials used in that growth process get removed during manufacturing, but trace amounts can remain in the final product.
Many flu vaccines are grown in fertilized chicken eggs, which is why they list egg protein (ovalbumin) as a trace ingredient. The amounts vary broadly, from 0.2 to 42 micrograms per milliliter depending on the vaccine. MMR vaccines also use chicken embryo cells during production, but the final product contains negligible egg protein, far less than flu shots.
Hepatitis B and HPV vaccines use baker’s yeast to produce their antigens, so trace yeast protein (up to 5% of the final product) can be present. Several other vaccines are grown in lab-cultured human or animal cell lines. The varicella (chickenpox) and hepatitis A vaccines, for instance, use a human cell line called MRC-5. After purification, only tiny fragments of cellular protein or DNA remain. Fetal bovine serum, a nutrient-rich liquid used to feed cells in culture, shows up as a trace ingredient in vaccines like MMR and rotavirus for the same reason.
Some manufacturing steps also use formaldehyde to inactivate viruses or antibiotics like neomycin to prevent bacterial contamination during production. These are largely removed in purification, leaving only residual traces in the finished vaccine.
mRNA Vaccine Ingredients
mRNA vaccines have a simpler ingredient list than many traditional vaccines, but they introduce one component that’s unique: lipid nanoparticles. The mRNA molecule is fragile and would be destroyed by your body’s enzymes before it could do its job. To protect it, the mRNA is wrapped in a tiny fat bubble made of four types of lipids.
The mixture follows a roughly 50:10:38.5:1.5 ratio of an ionizable lipid (the one that helps the particle fuse with your cells), a helper phospholipid, cholesterol (which adds structural stability), and a polyethylene glycol (PEG) lipid that keeps the particles from clumping together. The Pfizer vaccine uses lipids called ALC-0315 and ALC-0159, while the Moderna vaccine uses SM-102 and PEG-DMG. Beyond these lipids, the rest of the vial contains buffer salts (like tromethamine and sodium acetate) and sucrose. No adjuvant is needed because the mRNA itself triggers a strong immune response.
PEG is the component that has drawn the most attention as a potential allergen in mRNA vaccines. It’s widely used in laxatives, skin creams, and other medications, and true allergic reactions to it are very uncommon.
Preservatives
Preservatives prevent bacterial or fungal contamination, mainly in multi-dose vials where a needle enters the same container multiple times. The most well-known vaccine preservative is thimerosal, an organic mercury compound. Thimerosal was removed from routine childhood vaccines in the United States in 2001. Today, it’s found only in some multi-dose flu vaccine vials, and thimerosal-free versions of the flu shot are widely available. Vaccines for measles, mumps, rubella, chickenpox, polio, and pneumococcal disease have never contained thimerosal. Single-dose vials and pre-filled syringes generally don’t need a preservative at all.
Potential Allergens to Know About
The proteins most often linked to vaccine allergies are egg protein, gelatin, and, less commonly, yeast protein and latex. Egg protein matters most for flu vaccines, which contain measurable amounts of it. If you have a severe egg allergy, egg-free flu vaccines grown in cell cultures (like Flucelvax) or made with recombinant technology (like Flublok) are available.
Gelatin causes more confirmed allergic reactions than any other vaccine ingredient, though the overall rate is still very low. Yeast allergies are a consideration for hepatitis B and HPV vaccines, which contain small amounts of yeast protein from the manufacturing process. Latex doesn’t appear in the vaccine liquid itself, but some vial stoppers or syringe plungers contain natural rubber latex, which can be a concern for people with latex allergies. Packaging labels indicate when latex is present.
How Vaccine Ingredients Are Regulated
Every ingredient in a licensed vaccine goes through regulatory review by the FDA. Manufacturers must characterize the identity, purity, potency, and stability of the active ingredient, and provide certificates of analysis for every inactive component. Residual impurities from the manufacturing process, including leftover cell proteins, DNA fragments, and chemical reagents, must be identified and quantified. Any ingredient of human or animal origin requires testing to confirm it’s free of contaminating infectious agents. The complete ingredient list for every licensed vaccine is publicly available on the FDA’s website and in the package insert that comes with each product.